Several former members of the Alternative for Germany (AfD) party received threatening letters from a group calling itself the "AfD Army Faction".

The messages were sent to former AfD MEPs who left the party in protest at its anti-immigrant stance, demanding that they give up their seats.

While there is no evidence last night that those behind the letters have any links to the AfD party leadership, German police are taking the threats seriously and have opened an investigation.

The AfD stormed to its best results in regional elections last week on a platform of opposition to Angela Merkel's 'open-door' refugee policy.

The party emerged as a eurosceptic group in 2013 in response to the eurozone crisis.

But it has drifted dramatically to the Right in response to the migrant crisis and its original leader was ousted in a coup last year.

Frauke Petry, its new leader, has called for the police to be given the power to shoot asylum seekers at the border.

The party has called for a "zero limit" on asylum seekers, and for German women to have three children each.

It surged to second place in elections in the state of Saxony-Anhalt last week and won seats in two other regional parliaments.

The AfD won seven seats in the European Parliament in 2014, when it was primarily an anti-Euro party. But five of those MEPs have defected and Ms Petry has called for them to give their seats back to the AfD.

One of the letters said: "We will take revenge for every vote you cost the AfD! Blood! Death to you!"

Another was signed with "Heil Hocke", a clear reference to Bjorn Hocke, a prominent AfD member.

Bernd Kolmel, one of the MEPs who received the letters, said: "They are written in the same style and typed with a typewriter. Someone has probably done them in his basement."

Meanwhile, an opinion poll has shown slightly rising support for Angela Merkel's migrant policy. The poll for ZDF television found 53pc of Germans believe the country can cope with the asylum seekers, up from 47pc last month.

The findings may be linked to a new EU deal to stop the flow of migrants into Europe.